1. Field of the Invention
The present invention relates generally to sensor measurements in automobile control systems and, more particularly, to a system for enhancing the precision of an analog sensor reading in an automobile control system.
2. Discussion of Related Art
Current automobile engines are internal combustion engines that use a mixture of fuel and air to generate their driving power. Complete fuel combustion produces only carbon dioxide and water as its products; however, the conditions within an engine do not correspond to the idealized requirements necessary to produce complete combustion. Incomplete combustion produces other products that may include: carbon monoxide, hydrogen gas, hydrocarbons, nitrogen gas, oxygen gas, and various nitrous oxides. Some of these gases are commonly found in the atmosphere and pose few or no health risks. Others can be toxic, creating a desire to reduce such toxic emissions.
The United States and many other countries have strict standards regulating the emissions from automobiles. Catalytic converters transform toxic chemicals into safer compounds. They convert CO, H2, and HC into CO2 and H2O and also convert nitrous oxides into nitrogen gas and oxygen gas before these gases are emitted from the automobile. Catalytic converters, however, do not completely convert toxic byproducts of incomplete combustion into less harmful substances before emission into the atmosphere. The higher the efficiency of the catalytic converter, the more toxic gases are converted into safer forms before they are emitted into the atmosphere. The efficiency of a catalytic converter relates directly to the composition of its intake gases, and the composition of the intake gases is determined by the combustion conditions, including the air/fuel mixture ratio input to the engine.
The mixture of fuel and air used in the combustion chamber of an engine is regulated through a feedback mechanism. A sensor is placed in the exhaust manifold to measure the oxygen content in the expunged gases. The oxygen content of the combusted mixture varies with respect to where the engine is operating in relation to the stoichiometric point. Typically, the operating point of the engine is called the stoichiometric air/fuel ratio, and this corresponds to the point where the exact quantity of fuel needed for completed combustion is added to the air flow. The stoichiometric point yields the most efficient catalyst operation and produces the least amount of toxic byproducts. The varying operating characteristics of the vehicle change the efficiency of the combustion process and require altering the current fuel flow to maintain engine operating at or near the stoichiometric point. The oxygen sensor output enables optimization of the fuel-air ratio fed into the engine. Optimizing the fuel-air mixture entering the engine changes the combustion conditions and achieves more complete combustion, thereby operating the engine closer to the stoichiometric point.
Oxygen sensors used in most vehicles provide a voltage output that varies in accordance with the amount of oxygen in the combustion product. An analog-to-digital (A/D) converter receives the oxygen sensor output and generates a digital value input to a digital microprocessor. The microprocessor controls the air/fuel ratio and constantly adjusts the mixture entering the combustion chamber in order to maintain the engine operating near the stoichiometric point. Constant adjustment is required because changing engine and environmental conditions alter the efficiency of the combustion process, even for a constant fuel-air mixture ratio. The voltage output of the oxygen sensor varies with the amount of oxygen found in the combustion products.
A typical oxygen sensor functions as a switching device. The switching device outputs less than 0.25 volts when the input air/fuel ratio to the engine is leaner than stoichiometric and outputs greater than 0.75 volts when input air/fuel ratio to the engine is richer than stoichiometric. Due to the physics of the chemical reaction within the oxygen sensor, output voltages are typically limited to less than 1.0 volts.
In the area of xc2x11 percent of stoichiometric, the output waveform is very steep. In the area outside xc2x11 percent of stoichiometric, the output waveform is nearly flat. Within the area of xc2x11 percent of stoichiometric, minor changes in the oxygen content found in combustion products result in significant changes in the output voltage of the oxygen sensor. Conversely, outside of xc2x11 percent of stoichiometric, even significant changes in the oxygen content in the combustion products result in predictably small changes in the output voltages of the oxygen sensor. The steep characteristic of the oxygen sensor in the stoichiometric region makes measuring the prevailing operating point difficult.
As discussed above, most controllers utilize a A/D converter to covert the analog output voltage of the oxygen sensor into a digital value for use by an electronic engine controller. A typical A/D converter converts a voltage range that varies between 0 and 5 volts into an 8-bit digital value for use by the engine controller. An 8-bit digit value can vary between 0 and 255, yielding 256 gradations or counts. The 256 counts in the typical A/D converter translate into approximately 0.0196 volts per count. Because the normal output of the oxygen sensor varies between a voltage range of 0 to 1 volts, only counts 0 to 51 of the 256 possible counts are utilized to determine the value of the analog signal received from the oxygen sensor. Thus, only approximately 20 percent of the total range of the A/D converter is utilized. This limited resolution reduces the level of oxygen sensor output detail input to the engine control system. This reduced resolution is particularly important in the critical zone around stoichiometric where minor variations in the oxygen content of the combusted products result in large variations in the voltage output by the oxygen sensor.
Thus, there is a need to improve the resolution of the oxygen sensor signal applied to the A/D converter in the engine control system.
A control system for regulating the fuel and air mixture used in an engine. The control system includes an engine producing drive power through combustion of fuel and air. An analog sensor connected to the engine monitors a concentration of gases produced through the combustion of fuel and air in the engine. The analog sensor generates an analog signal that varies in accordance with the concentration. The output signal is within a first predetermined voltage range. An amplifier receives the analog signal and amplifies the analog voltage to generate an amplified signal. The amplified signal is within a second predetermined voltage range, wherein the second voltage range is greater than the first voltage range. An analog-to-digital (A/D) converter receives the analog signal and generates a digital signal that varies in accordance with the amplified signal. The A/D converter converts input voltages varying within the second voltage range. A microprocessor receives the digital signal from the A/D converter and produces a mixture signal that varies in accordance with a desired fuel and air mixture, wherein the desired mixture varies in accordance with the analog signal.
These and other advantages and features of the present invention will become readily apparent from the following detailed description, claims and drawings.